[l3][scale issue] unrestricted hosting routers in network node increase service operating pressure

[LIU Yulong]

[l3][scale issue] unrestricted hosting routers in network node increase service operating pressure

Related problem was reported here: https://bugs.launchpad.net/neutron/+bug/1828494
These issues have same background, unlimited router creation for entire cluster,
"""
Every tenant may create free routers for doing nothing. But neutron will create many resource for it, especially the HA scenario, there will be namespaces, keepalived processes, and monitor processes. It will absolutely increase the failure risk, especially for agent restart.
"""

So this bug is aimming to add config or mechanism which will add some conditions for l3-agent to indicate whether to create resources in network nodes. For now, the condition can be whether there are resources under the router, aka if there is no compute (virtual machine or baremetal) related port under the router, the l3-agent in network node will host nothing for this router.